1. Field of the invention
The present invention relates generally to an apparatus and a method for inspecting the degradation of a gas nozzle. The nozzle is of the type that can produce a whistle when a gas is blown through the nozzle.
2. Description of the prior art
In recent years, the plasma arc cutting process has gained wide acceptance as a cost-effective means of cutting steel, aluminium, and stainless steel. The process was invented in 1955 and was originally intended for cutting aluminium, although approximately 80 percent of plasma arc cutting is now used for cutting carbon steel below 1/2 inch thick. Many of today's plasma arc cutting systems are highly automated workcells that can be integrated into a CAD-CAM environment. Like other cutting process, plasma arc cutting works by locally applying heat and momentum on the workpiece to melt it and expel the molten material. A cutting gas, such as nitrogen, flowing out of a nozzle at high velocity provides the momentum while an electric arc, burning through the same nozzle between a tungsten electrode and the workpiece, provides the heat. This arc is called a "transferred arc" because it is sprung from a pilot arc initially struck between the electrode and the nozzle.
The orifice of the nozzle must sustain the high-velocity flow of the cutting gas, which is heated to a plasma of more than 30,000.degree. C. Thus the nozzle is a very critical component of any plasma arc cutting system because it tends to wear rapidly under such extreme operating conditions, and the resulting change in the geometry of the orifice affects the hydrodynamic behavior of the plasma jet, which consequently degrades the quality of the cut.
It is known that tip life is very dependent on operating conditions such as torch stand-off, cutting gas mixture, and current density. The following conditions and their causes are examples of nozzle defects: general wear, which consists of axisymetric erosion of the orifice and occurs after prolonged use and usually on the exit edge; pitting, a chipping of portions of the inner orifice caused by too-low-cutting gas flow, gas contamination, loss of gas swirl, a swirl being forced on the gas flow to reduce nozzle wear, or excessive striking of the pilot arc; blow out, an extended case of pitting caused by overpowering which consists of a too high current density for the nozzle; blowback, which usually occurs during piercing when cut material is blown back up the tip; and calcium build up which is a problem associated with water-injection plasma cutting.
The detection of these conditions is currently performed visually, either through direct inspection of the tip or indirectly when a deterioration of cut quality or unreliable arc initiation is observed. No sensing technique that could replace human intervention exists or has been reported. This is a problem, particularly with large automated plasma arc cutting systems, where a faulty tip can cause substantial waste of time and material due to bad cuts.